Electrostatic image transfer processes and apparatus therefor



R. M. SCHAFFERT TATIC IMAGE Sept. 8, 1964 3,147,679 ELECTROS TRANSFER PROCESSES AND APPARATUS THEREFOR 2 Sheets-Sheet 1 Filed Dec. 18, 1961 AIDING POTENTIAL oPPo'smc AIDING F IG.4

INVENTOR ROLAND M. SCHAFFERT ATTORNEY Sept. 8, 1964 Filed Dec. 18. 1961 AND APPARA SCHAFFERT 2 Sheets-Sheet 2 V I B C /4BI D IA) M I g 7/ '7 5 E Z V D E Z z I K A- b 8 9 10 H 12 15 i4 i5 15 17 13 CAP WIDTH (WCRONS) GAP VOLTAGE A BREAKDOWN VOLTAGE 3 20 CAP W|DTH(M|CRONS) United States Patent' 3,147,679 ELECTROSTATEC WAGE TRANSFER PRUCESES AND APPARATUS THEREFGDR Roland Michael Sehadert, Saratoga, Calif, assignor to International Business Machines florporation, New

York, N.Y., a corporation of New York Filed Dec. 18, 1961, Ser. No. 159,892 4 Claims. (Cl. 95-13) The invention relates to electrophotography and electrostatic printing, and it particuarly pertains to the transfer of latent electrostatic images from an electrophotographic plate or other medium to dielectric material.

The transfer of latent electrostatic images from one surface to another, as for example from an electrophotographic plate to a dielectric surface, provides a method of electrostatic printing or copying free from the steps of plate and drum cleaning, thereby eliminating the need for cleaning devices, and consequently increasing the life of plates and drums and reducing the maintenance thereof. However, the published prior art processes of electrostatic image transfer have not been found suitable for practical applications of the teaching. Despite some rather remarkable developments in the field, .processes known in the art for the transfer of Electro Static Images (an art at times referred to by the acronym TESI) have not found practical application in commercial electrophotographic or electrostatic printing as yet.

It is known to those versed-in the art that the inherent resolution and sharpness of electrostatic images, such as those formed by the electrophotographic process on amorphous selenium plates, are extremely high. However, the prior art methods of electrostatic image transfer permit migration of electric charges across a relatively wide air gap which, due to image spread, results in a transferred electrostatic image with poor resolution and lacking in sharpness. Furthermore, theseprior methods require maintaining the width of the air gap within closely controlled critical limits, since variation of a few microns will result in non-uniform charge transfer. Thus, the transferred image will be non-uniform in strength resulting in density variations and generally poor quality of the developed image.

There is a process of electrostatic image transfer described in .a copending US. patent application of Harold Clinton Medley and Roland Michael Schaffert, Serial.

No. 127,725, filed on July 28, 1961. According to that patent application latent electrostatic images are produced on dielectric material by electric charge transfer from surfaces of photoconductive insulating material or another dielectric material upon which electrostatic images have been formed by known techniques. For example, a photoconductive insulating layer is sensitized in darkness by electrically charging the surface uniformly to a predeterminad potential of given polarity with respect to a conductive surface backing element, and an image of the desired subject is optically projected onto the photoconductive insulating layer to produce a charge pattern corresponding to the image of the desired subject. The manner in which the initial electrostatic image is formed is not a part in itself of the invention, however, other methof dielectric material to which the electrostatic image is i to be transferred is backed with a conductive element and the dielectric surface electrically charged uniformly at a polarity opposite to the polarity of the charge of the iniitial electrostatic image. The charge pattern is then transferred by placing the charged layer of dielectric ma- 3,147,679 Patented Sept. 8, 1954 terial in contact with the surface layer containing the initial electrostatic image in darkness, and applying a direct potential from an external source between the two conductive backings of the respective layers. The potential is applied between the conductive backing elements such that the lead attached to the conductive backing of the dielectric layer is of the same polarity as the charge originally placed on the dielectric layer. The direct potential applied between the conductive surface elements after the materials have been placed in contact is preferably obtained from a variable source, so that it can be adjusted from zero to a relatively high value, up to several thousand volts.

This process, while capable of excellent results, has one drawback remaining in that it is still difficult under some conditions to prevent spark discharges and premature transfer on bringing the image charged and the dielectric surfaces into contact.

An object of the invention is to insure image charge transfer only after the image charged and dielectric surfaces have been placed in virtual contact.

Another object of the invention is to prevent the transfer of a latent electrostatic image, or portions thereof, from an electrostatic image-bearing surface (such as a photoconductive-insulating surface) to a dielectric surface before these surfaces are brought into virtual contact for effecting the transfer thereafter thereby attaining greater accuracy of reproduction of the original electrostatic image with a high degree of resolution and sharpness, and eliminating the necessity of maintaining an air gap of critical dimensions.

A further object of the invention is to transfer a latent electrostatic image to a dielectric surface wherein the resultant image areas on the dielectric surface are to be of one polarity of electric charge and the background areas are to be of the opposite polarity of electric charge, for producing greater contrasts and cleaner development of the image, without discharging prior to the bringing together of the image bearing surface and the dielectric surface.

According to the invention the transfer of latent electrostatic images to dielectric material by electric charge transfer from surfaces of photoconductive or insulating material upon which electrostatic images have been formed by known techniques is prevented by subjecting the materials to electric stresses opposing charge transfer as the materials are brought together. The desired charge transfer is thereafter effected while the image bearing material is in virtual contact with the dielectric material or upon separation of the materials, by altering the electric stresses to favor charge transfer.

Inthe practical case the photoconductive or other image charge bearing surface-and the dielectric material are backed by conductive backing surface elements. Charge transfer opposing stresses are set up by applying a potential or potentials of polarity and value between the conductive backing surface elements opposing charge transfer until the surfaces are brought into contact. At this time the external potential interposed between the surfaces is altered either to zero or to some predetermined potential of polarity opposite to that of the transfer opposing potential. In general the charge transfer aiding potential will be that effecting transfer across the shortest gap distance possible in accordance with Paschens law for the breakdown of gas in an electric field.

Apparatus for automatically and continuously carrying out a process according to the invention comprises in one form a conventional rotatable drum having a peripheral conductive surface element over which there is a layer of photoconductive material, conventional means for uniformly charging the photoconductive layer at given polarments of the electrostatic image bearing drum and one of the rollers are connected to opposite terminals of a charge transferaiding potential source, which is adjustable from zero to several thousand volts direct potential. In accordance with the invention the conductive surface element of the remaining roller orrollers and the image bearing drum are connected to opposite terminals of a charge transfer opposing potential source adjustable from zero to several hundred volts and of polarity reversed with respect to that of the opposing potential source.

Thus a portion at least of the charge is transferred while the surfaces are in virtual contact and charge transfer prior to the surfaces beingbrought into contact is prevented according to the invention by applying the charge opposing potential across the backing surface elements as the film material is brought to the drum. Further according to the invention charge transfer upon or after separation of the film material from the drum is prevented by again applying the charge opposing potential by means of an additional backing surface roller. Conventional means for discharging the photoconductive layer after transfer of the image therefrom to the dielectric film material and conventional means for developing the image transferred to the dielectric material are provided,

In order that full advantage of the invention may be readily obtained in practice, preferred embodiments thereof,' given by way of example only, are described in detail hereinafter with reference to the accompanying drawing forming a part of the specification, and in which:

FIG. 1 illustrates an example of apparatus as arranged for a particular step in the transfer of an electrostatic image according to the invention;

FIGS. 2 and 3 are graphs providing data and indicating conditions important to an understanding of the invention;

and

FIG. 4 illustrates an example of apparatus for carrying outthe process of the invention in an automatic and continuous mode of operation.

FIG. 1 depicts an example of the essentials of apparatus necessary for carrying out. a method according to the invention for forming alat'ent electrostatic image on dielectric material. An electrostatic image-corresponding to a desired document is formed by conventional means, for example, as shown and described in the aforementioned copending US. patent application Serial No. 127,725, on a xerographic plate comprising a conductive substrate 12 coated with photoconducting material 14 such as amorphus selenium. -A conventional corona charging ,unit, energized by direct potential supply capable of delivering between 4000 and 9000 volts, is swept across the surface of the selenium layer 14 in darkness. The image of 'a document is then projected onto the xerographic plate 10, within some conventional arrangement (not shown) for excluding ambient light, discharging the positive charge immediately above the selenium layer l4and leaving the positive charges as shown, only in the areas corresponding to dark areas of the document.

A desired dielectric material 30 is backed by a conducting backing surface element 32 and sensitized by charging negatively to a uniform potential by sweeping with another conventional corona charging unit connected to the direct potential supply. The charge placed on the dielectric material 30 is always of opposite polarity to the charge placed on the plate10. Amorphous selenium is given as phous selenium can be charged negatively and electrostatic 'closing another transfer opposing switch 35b.

face smoothness.

image transfer accomplished according to the invention, it is well known that this material functions best when positively charged. It should be understood, however, that it is clearly possible to form an electrostatic image with either polarity and transfer the same to a dielectric material charged to the opposite polarity.

As shown in FIG. 1 the backing surface 12 and the image bearing surface 14 of the plate 10 are subjected in accordance with the invention to a charge transfer opposing electric field obtained from an opposing potential supply 34a by closing a switch 35a. Likewise the dielectric material 30 and the associated backing surface element 32 are subjected to charge transfer opposing electric 'field stress by applying another charge transfer opposing potential therebetween obtained from another charge transfer opposing potential supply 34b and applied by Still in darkness, the image charged selenium 14 of the plate 10 and the backed dielectric material 30 are then brought into virtual contact with the transfer opposingpotentials applied as shown to prevent any premature charge transfer or spark discharge. The conductive backing elements are then disconnected from the charge transfer opposing potential supplies 34a, 34b by opening the opposing potential switches 35a, 35b. The image is transferred according to the invention when the conductive backing elements 12 and 32 are maintained at a predetermined direct potential obtained from a transfer aiding potential supply 36 by closing aiding potential switches 37a, 37b.

The transfer aiding supply must be capable of supplying direct potential in a range from'zero to several thousand volts as will be described hereinafter. In the special case of interconnection at zero potential a switch (not shown) may be thrown to bypass the transfer aiding supply 36,

if desired.

It is obvious, of course, that the individual potential supplies 35a and 35b may be replaced by a single supply capable of producing the sums of the separate potentials, in which case the ground connection may be made to either backing surface element 12 or 32 as desired.

Examples of dielectric material which have been used successfully-in practicing the invention are: Polyethylene glycol terephthalate, which is a polyester most commonly known by the registered trademark Mylar, polystyrene, polyethylene, andstyrene-butadiene copolymers. Alumi- 'nized Mylar film has been used as a material having a contice, the gas will be air at atmospheric pressure. This air fills the gap between the electrostatic image surface and the surface of the dielectric material to which the image is to be transferred. As the two surfaces are brought together the gap between decreases frorn a relatively large value to a very thin film even when the surfaces are brought into virtual contact.

The gas film thickness at contact will depend upon the degree of sur- Surfaces polished to optical standards and placed in contactare still separated by gaps of the order of 3 l0 cm. The gas film between a smooth amorphous selenium surface in contact with a smooth dielectric film is estimated to be in the range of 0.5 to 1.0 micron. j

According to Paschens'law the breakdown voltage of gas in an electric field is a linear function of the product of the gas pressure and the distance between the electrodes." This law holds for values for the distance-pressureproduct which are greater than about 5 mmmr mm.

age increases sharply for gases at low pressures.

. contact.

of mercury in air. Below this value, the breakdown volt- However, for relatively high pressures (for example, normal atmospheric pressure), the sharp upward turn of the breakdown curve is not observed. At these relatively highpressures, the gaps (below the Paschen minimum) separating the surfaces are very small, and discharge is due primarily to field emission because of the very high electric fields. For instance, the breakdown curve in air for very srfiall gaps (less than about 8 microns for air at 760 mm. of mercury) is a downward sloping curve due to electron emission from the surface forming the gap. Such a curve is very useful in practice when it is desired to transfer charges across extremely small gaps.

It will be appreciated that charge transfer is preferred across a small gap to avoid the image spread which occurs when the charges are transferred across a relatively wide gap. In all of the techniques of electrostatic image transfer, the image surface and the dielectric surface are brought together and then, subsequently, separated. During these manipulations it is desired that conditions be avoided that produce air breakdown discharges when the gap between the surface is large. Spark discharges are to be avoided under any conditions and according to the invention, transfer is made under conditions that will produce silent discharge when the surfaces are very close together. Preferably according to the invention while the surfaces are being brought together, bias potentials are applied opposing charge transfer and then transfer is effected while the surfaces are in virtual contact by altering the potential in value and polarity to-provide a voltage across the gap which will produce dis charge by field emission. Further according to the invention the charge transfer opposing potential is reapplied under certain conditions made apparent hereinafter, to prevent any additional possible charge transfer as the surfaces are separated.

Reference now is made to FIG. 2 in which the heavy solid black curve M represents the minimum voltage for discharge as a function of gap width for air at atmospheric pressure. The nature of this curve M will be different for different gases and different pressures; however, the. slope of the critical field emission line will be independent of the type of gas, depending only on the nature of the surfaces forming the gap. The plateau portion of the curve (at approximately 360 volts) is an extension of the Paschen minimum; the upward sloping line of the rightof the plateau which is the lower portion of the normal Paschen curve for breakdown of the gaseous discharge; and the steep sloping line portion to the left of the plateau is the critical field emission line, which in FIG. 2 has been drawn to correspond to a slope of 10 volts per cm. Shown also in FIG. 2 are curves representing gap voltage plotted against gap'width.

for several different conditions of applied voltage, charge voltage of the initial electrostatic image, charge voltage of the dielectric surface, and thickness and dielectric constant of the dielectric materials. The light solid curves A, B, C and D indicate gap'voltages in the regions of the electrostatic images and the light broken curves A, B, C and D are for gap voltages in the regions of the background areas.

The curve M in FIG. 2 is shown extended in FIG. 3 wherein the light solid curves A, B", C and D" represent the relationship of gap voltage and gap Width upon the application of transfer opposing potentials for preventing premature charge transfer as the charge image bearing and dielectric surfaces are brought into virtual It will be immediately noted that all of these curves A D" fall below the Paschen minimum curve M, thus insuring that no discharge occurs between the two surfaces during the time that they are being brought together.

Several examples of image transfer according to the invention willnow be described in order to more clearly set forth a manner of practicing the invention.

Example 1 Thickness of image layer 14 microns 12.6 Thickness of dielectric layer 30 do 36 Dielectric constant of layer 14 6.3 Dielectric constant of layer 30 3.0 Charge voltage of electrostatic image volts +500 Charge voltage on the dielectric layer do -l000 Opposing voltage of supply 34a do 400 Opposing voltage of supply 34b do 400 Total opposing voltage do 800 Aiding voltage do +1500 These conditions are illustrated by curves A and A in FIG. 2 and by curves A in FIG. 3. It will be noted that both curves A and A are above the critical field emission line portions of the curve M. Thus, electric charges will transfer while the surfaces of layers 14 and 30 are in virtual contact. Since the curve A for the image area is higher than that of curve A for the background area, a greater amount of charge will transfer in the image area than in the background.

A process for accomplishing image transfer under the conditions set forth above is as follows:

A- photoconductive layer 14 is given a positive surface charge of 500 volts with respect to the conductive base 12 and an electrostatic image is produced on the layer 14-. The dielectric layer 30 is given a negative surface charge of 1000 volts with respect to the conductive base 32.

With the surfaces 14 and 30 widely separated, the transfer opposing switches 35a, 35b are closed to apply a transfer opposing potential of 800 volts between the conductive backing surface elements 12 and 32. This transfer opposing potential value is derived from the curves in FIG. 3 and the foregoing data to maintain a potential resultant from the individual charge potentials and the'charge transfer opposing potential, lying beneath the minimum curve M, whereby no transfer or spark discharge can take place.

The layers 30 and 14 are then brought into virtual contact and the switches 35a, 35b opened. The charge transfer switches 37a and 3711 are then closed momentarily to apply 1500 volts direct potential, after which the switches 37a and 37b are opened and the transfer opposing switches 35a and 35b are again closed before the two surfaces 14 and 32 are separated. After separation of the surfaces the switches 35a and 35b are re opened and the transfer process is now complete.

Is is found after the-above procedure,.that an electrostatic charge pattern corresponding to the initial image has been formed on the dielectric layer such that the image and background charges are of opposite polarity; the image areas having a positive polarity charge of approximately 260 volts, and the background areas having a negative polarity charge of approximately volts. This condition is particularly desirable for attaining high contrast in development, for example, with a developing technique such as the powder cloud method where the image surface is subjected to the cloud of aerosol particles electrically charged with negative polarity.

Furthermore, it will be noted that charge transfer took place only while the surfaces were in virtual contact with the gap width in the range of 0.5 to 1.0 micron. Thus, this procedure is capable of achieving high electrostatic resolution in the transferred image, and there is very little loss of resolution during the transfer step. It is known that the maximum resolution attainable in electrostatic image transfer is an inverse function of the gap width, being approximately proportional to the reciprocal of the gap width. It has been found that when transfer takes place across a gap width of one micron, a maximum resolution of 200 lines per mm. is possible.

a negative polarity of 200 volts.

7 Example 2 Thickness of dielectric layer 14 microns 25 Thickness ofdielectric layer 30 n do 25 Dielectric constant of layer 14 6.3 Dielectric'constant of layer30. 3.0 Chargevoltage of image volts +700 Charge .voltage on dielectric do 300 Transfer opposing voltage (total) do 350 Transfer aiding voltage do 1000 These conditions are represented by the curves B and B in FIG. 2 and curve B" in FIG. 3. It will be noted from curve B that as in the case of Example 1, the image charge is transferred by field emission while the surfaces are in virtual contact. Thus, a high maximum resolu tion is attainable in the transferred image. It will also be noted from curve B that only a small amount of charge is transferred in the background areas under these conditions. Curve B", falling below the Paschen minimum curve M, indicates that premature charge transfer is prevented. V

The procedure for transferring electrostatic images with the conditions of Example 2 is essentially the same as for Example 1, except that the voltages applied are of different values.

It is found that after separation of the surfaces, an electrostatic image corresponding to the initial image is formed on the dielectric layer 30, such that the image areas are charged to a positive polarity of approximately 190 volts, and the background areas are charged to a negative polarity of approximately 275 volts.

Example 3 and curve C in FIG. 3. It will be noted that with these conditions charge will transfer in the image areas by gaseous discharge at about 6.7 microns whereas no discharge will take place in the background areas as can be seen from curve C, which does not cross the minimum curve M.. Also from curve C" in FIG. 3, it will be evident that premature charge transfer is prevented.

The process for transferring electrostatic images under these conditions is somewhat different from the processes of Examples 1 and 2. In this case, the image layer 14 is charged positively to 800 volts and an electrostatic image formed on this layer in the usual manner. A negative charge of 200 volts 'is then applied to the dielectric layer 30. A charge transfer opposing potential of 400 volts is applied and the surfaces are then brought together, after Which-the conductive backing surfaces 12 and 32 are electrically interconnected at substantially zero potential, preferably at ground potential as shown. This connection is'maintained as the surfaces are separated.

It is found that with this procedure and these conditions an electrostatic image is formed on the layer 30 such that the image areas are charged to a positive polarity of 195 volts, and the background remains charged to Since transfer takes place at 6.7 microns, the maximum resolution attainable for electrostatic image transfer in this manner is about 30 lines per mm.

L These conditions give rise to curves C and C in FIG. 2

' 'Example4 Thickness of layer 14 microns 50.4 Thickness of layer 30 do 48.0 Dielectric constant of layer 14 6.3 Dielectric constant of layer 30 3.0 Charge voltage of the image volts +700 Charge voltage of the dielectric layer 30 do 500 Transfer opposing voltage do 500 Transfer aiding voltage do 0 These conditions result in curves D and D in FIG. 2 and curve D in FIG. 3, from which it will be noted that transfer in the image areas takes place by gaseous discharge at a gap width of 11.0 microns; whereas no charge is transferred in background areas. Also no charge is transferred during the time that transfer opposing potential is applied.

The procedure here is the same as in Example 3, except that the voltages applied to the surfaces are different. After separating the surfaces it is found that an electrostatic image has been formed on the dielectric layer 30 such that the image areas are essentially neutralized, whereas the background areas remain charged to a negative polarity of 500 volts. The maximum resolution attainable in this case is about 18 lines per mm.

Examples 1 and 2 are particularly suited for transferring electrostatic images of micro-image size, whereas Examples 3 and 4 are suitable for transferring images of intermediate and micro-image size; for example, images normally readable without optical aids.

The above examples all utilize initial electrostatic images of positive polarity. It will be appreciated that images of negative polarity could be transferred by similar procedures and processes, in which case the dielectric layer 30 would be charged to positive polarity, instead of negative as shown in and described hereinbefore.

Thus far the techniques of the invention have been described as a stepwise process performed with flat surface structural elements. Continuous processing, using an image retaining drum instead of the electrophotographic plate, is possible With the techniques, and an example of such an arrangement according to the invention .is shown in FIG. 4. The essentials are shown in this illustration, itbeing understood that conventional methods and structures for transporting the various components of the apparatus, shielding the charged areas from light or electrostatic fields, and the like, are readily apparent to those skilled in the art.

A drum 10 conveniently completely metallic, but at least having a conductive peripheral surface element 12' maintained at ground potential, as shown, and having a charge image recording layer; for example, of amorphous selenium 14 thereon, is arranged in a light tight housing indicated symbolically by the lines 40, the upper and rear parts of which are hinged for access in feeding documents and servicing the unit.

Images on a continuous web of documents 42 (or on single documents inserted and removed by hand, one after the other, into a feeding slot 44), illuminated by a synchronized slit exposure system indicated only schematically by a pair of lamps 46, are projected in succession by an optical system indicated only by a schematic lens 47 onto the selenium layer 14'. The layer 14' is charged uniformly by a conventional corona charging unit 48 energized by a direct potential supply 50 to form charge images on the selenium drum 10 in more or less conventional manner. A web 30 of dielectric material unwinding from a supply reel 54 and winding on a takeup reel 56 is carried over guide rollers as necessary. At a point where the web 30 passes over a conductive roller 58 which is maintained at fixed reference potential, preferably ground, the web is given a uniform negative charge by means of a corona charging unit 61) energized by a direct potential supply62 of polarity opposite to that of the previous mentioned potential supply 50. A suitably 9- housed lamp 64 is arranged todischarge any image charge remaining on the selenium coating 14' after charge transfer.

Conductive rollers 32', 32" and 32" are arranged (by means not shown) to urge the web 30' into contact with the xerographic drum 1&9, and'to back the web 30' at the areas where the control of the charge transfer processes according to the invention is' effected. These backing rollers 32'32' have at least peripheral conductive surface elements forming the backing conductive surface element of the dielectric web 30' during the transfer process and preferably connected to the shafts on which the backing rollers 32'32"' rotate, as the conductive surface elements on the backing rollers 3232"' are electrically insulated from the remainder of the structure for connection todirect potential power supplies. As shown, two rollers 32' and 32" are connected to charge transfer opposing potential source 72 delivering from zero to several hundred volts and another roller 32" is connected to a charge transfer aiding potential source 74, delivering from zero to several thousand volts direct potential. The switching from one potential to the other during the charge transfer process according to the invention is effected by the translation of the web 3% with respect to the drum and the rollers 3232".

As the negatively charged film material web 30' comes into contact with the first roller 32' a charge transfer opposing potential is applied according to the invention between the conductive surface element 12 of the drum 10' and the roller 32' preventing charge transfer across the. narrowing gap between the image charge surface 14' and the web 305. The charge transfer is impelled as the web 30' passes between the drum 1t) and the second roller 32' by the charge transfer aiding potential applied between the drum 10 and the roller 32". Under conditions of complete image charge transfer from the surface 14 to the web 30 the latter may be led directly to a developing station. However, there are conditions of transfer, as discussed hereinbefore, under which additional and undesirable charge willbe transferred as the web 39 is separated from the charged surface 14' across the winding gap. This undesirable transfer is prevented further according to the invention by the third roller 32 which is arranged to reapply the charge transfer opposing potential from the opposing potential supply 72 until the gap has rendered far beyond the critical dimension.

The resulting charge pattern is developed by a conventional developing means; shown only schematically in the drawing as a powder charge unit 80 comprising a suitable powder 81 in a hopper 82 cascading down onto a pile fabric covered roller 84, the overflow powder being caught in a bin 83 and by a suitable arrangement (not shown) returned to the hopper 81 for later use. The image is developed on the web 30 on the application of the powder 81 to the film 3th by velvet covered roller 86 and the electrostatic force, applied through a conducting backing roller 83 and a developing potential supply 90. The image developed is then fixed by means of a heat fusing unit 92 according to known techniques.

By suitable arrangement, the lens 47 can be shuttered; the corona discharge unit 48 can be disconnected and the discharge lamp 64 can be extinguished, so that the residual charge on the selenium layer 14' can be used to transfer duplicate images as desired. Conceivably the arrangement can be so shuttered in known fashion that an image on the selenium coating 14' can be discharged in a local area only and a new portion inserted thereat for spot updating of the information record.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, and various alterations have been suggested, it should be understood that those skilled in the art may effect still further changes in the form and details without departing from the spirit and scope of the invention.

The invention claimed is:

1. The process of transferring an electrostatic image of a given polarity from the surface of an image-bearing medium having a conductive backing element to the surface of a dielectric image-receiving medium also having a conductive backing element, said surfaces initially being spaced apart from one another, said process comprising the steps of:

distributing on the surface of said dielectric imagereceiving medium a predetermined level of charge of a polarity opposite said given polarity;

moving said image-bearing and said image-receiving surfaces into virtual contact while said conductive backing elements are subjected to a charge transfer inhibiting voltage differential which places on said conductive backing element of said image-receiving medium a charge which is of said given polarity and which has a magnitude sufficient to maintain the total voltage across the gap between said surfaces below the minimum breakdown value; and

removing said charge transfer inhibiting voltage differential, after virtual contact has been established between said surfaces, to permit the transfer of said electrostatic image from said image-bearing medium to said image-receiving medium.

2. The process of transferring an electrostatic image of a given polarity from the surface of an image-bearing medium having a conductive backing element to the surface of a dielectric image-receiving medium also having a conductive backing element, said surfaces initially being spaced apart from one another, said process comprising the steps of distributing on the surface of said dielectric imagereceiving medium a predetermined level of charge of a polarity opposite said given polarity;

moving said image-bearing and said image-receiving surfaces into virtual contact while said conductive backing elements are subjected to a charge transfer inhibiting voltage differential which places on said conductive backing element of said image-receiving medium a charge which is of said given polarity and which has a magnitude sufficient to maintain the total voltage across the gap between said surfaces below the minimum breakdown value;

removing said charge transfer inhibiting voltage differential, after virtual contact has been established between said surfaces, to permit the transfer of said electrostatic image from said image-bearing medium to said image-receiving medium; and

moving said image-bearing and said image-receiving surfaces out of virtual contact while said conductive backing elements are re-subjected to said charge transfer inhibiting voltage differential.

3. A device for producing an electrostatic image on a dielectric medium, comprising:

a rotating drum including an electrically conductive cylinder having a layer of photoconductive material covering a portion of its outer surface; v

means for uniformly distributing a predetermined level of charge of a given polarity over the surface of said photoconductive layer;

means for projecting an optical image on the charged surface of said photoconductive layer, a corresponding electrostatic image thereby being formed on said surface;

first and second guide rollers having electrically conductive outer surfaces rotatably positioned adjacent said drum, said rollers having their respective axes of rotation parallel to the axis of said drum;

a web of dielectric material pressed against a portion of the surface of said drum by said guide rollers, said web moving into virtual contact with said drum surfaces as it passes over said first guide roller and being separated from said drum surfaces as it passes over said second guide roller, there being a minute 11 but finite gap between said web and said drum surface at points of virtual contact therebetween; means for charging said web to a predetermined potential of a polarity opposite to said given polarity prior to initial virtual contact between said web and said drum; I

a voltage supply electrically connected to the outer surface of said first guide roller, the potential applied by said voltage supply to said first guide roller being of the same polarity as said given polarity and having a magnitude sufficient to maintain the total voltage across the gap between said web and said drum surface, as the latter are moving into virtual contact, below the minimum breakdown value, the transfer of charge between said web and said drum surface thereby being inhibited; and

electrical means for maintaining said conductive cylinder of said drum and the outer surface of said second guide roller at substantially the same potential, whereby the electrostatic image on said photoconductive layer is transferred to said web.

4. A device for producing an electrostatic image on a dielectric medium, comprising:

a rotating drum including an electrically conductive cylinder having a layer of photoconductive material covering a portion of its outer surface;

means for uniformly distributing a predetermined level of charge of a given polarity over the surface of said photoconductive layer;

means for projecting an optical image on the charged surface of said photoconductive layer, a corresponding electrostatic image thereby being formed on said surface; i

first, second, and third guide rollers having electrically conductive outer surfaces rotatably positioned adjacent said drum, said rollers having their respective axes of rotation parallel to the axis of said drum;

a web of dielectric material pressed against a portion of the surface of said drum by said guide rollers,

' said web moving into virtual contact with said drum surface as it passes over said first guide roller and as it passes between said second guide roller and said drum surface, said web being separated from said drum surface as it passes over said third guide roller, there being a minute but finite gap between said web and said drum surface at points of virtual contact therebetween;

means for charging said web to a predetermined potential of a polarity opposite to said given polarity prior to initial virtual contact between said web and said drum;

a first voltage supply electrically connected to the outer surface of said first guide roller, the potential applied by said first voltage supply to said first guide roller being of the same polarity as said given polarity and having a magnitude sufiicient to maintain the total voltage across the gap between said web and said drum surface, as the latter are moving into virtual contact, below the minimum breakdown value, the transfer of charge between said web and said drum surface thereby being inhibited;

a second'voltage supply electrically connected to the outer surface of said second guide roller for creating a charge transfer aiding potential between said photoconductive layer and said web to effect the transfer of charge therebetween; and

means for applying the charge transfer inhibiting potential of said first voltage supply to the outer surface of said third guide roller for opposing further charge transfer during separation of said web from said photoconductive layer, thereby producing a latent negative of said electrostatic image on said web.

References Cited in the file of this patent UNITED STATES PATENTS Hall Apr. 2, 196 3 

3. A DEVICE FOR PRODUCING AN ELECTROSTATIC IMAGE ON A DIELECTRIC MEDIUM, COMPRISING: A ROTATING DRUM INCLUDING AN ELECTRICALLY CONDUCTIVE CYLINDER HAVING A LAYER OF PHOTOCONDUCTIVE MATERIAL COVERING A PORTION OF ITS OUTER SURFACE; MEANS FOR UNIFORMLY DISTRIBUTING A PREDETERMINED LEVEL OF CHARGE OF A GIVEN POLARITY OVER THE SURFACE OF SAID PHOTOCONDUCTIVE LAYER; MEANS FOR PROJECTING AN OPTICAL IMAGE ON THE CHARGED SURFACE OF SAID PHOTOCONDUCTIVE LAYER, A CORRESPONDING ELECTROSTATIC IMAGE THEREBY BEING FORMED ON SAID SURFACE; FIRST AND SECOND GUIDE ROLLER HAVING ELECTRICALLY CONDUCTIVE OUTER SURFACES ROTATABLY POSITIONED ADJACENT SAID DRUM, SAID ROLLERS HAVING THEIR RESPECTIVE AXES OF ROTATION PARALLEL TO THE AXIS OF SAID DRUM; A WEB OF DIELECTRIC MATERIAL PRESSED AGAINST A PORTION OF THE SURFACE OF SAID DRUM BY SAID GUIDE ROLLERS, SAID WEB MOVING INTO VIRTUAL CONTACT WITH SAID DRUM SURFACES AS IT PASSES OVER SAID FIRST GUIDE ROLLER AND BEING SEPARATED FROM SAID DRUM SURFACES AS IT PASSES OVER SAID SECOND GUIDE ROLLER, THERE BEING A MINUTE BUT FINITE GAP BETWEEN SAID WEB AND SAID DRUM SURFACE AT POINTS OF VIRTUAL CONTACT THEREBETWEEN; MEANS FOR CHARGING SAID WEB TO A PREDETERMINED POTENTIAL OF A POLARITY OPPOSITE TO SAID GIVEN POLARITY PRIOR TO INITIAL VIRTUAL CONTACT BETWEEN SAID WEB AND SAID DRUM; A VOLTAGE SUPPLY ELECTRICALLY CONNECTED TO THE OUTER SURFACE OF SAID FIRST GUIDE ROLLER, THE POTENTIAL APPLIED BY SAID VOLTAGE SUPPLY TO SAID FIRST GUIDE ROLLER BEING OF THE SAME POLARITY AS SAID GIVEN POLARITY AND HAVING A MAGNITUDE SUFFICIENT TO MAINTAIN THE TOTAL VOLTAGE ACROSS THE GAP BETWEEN SAID WEB AND SAID DRUM SURFACE, AS THE LATTER ARE MOVING INTO VIRTUAL CONTACT, BELOW THE MINUMUM BREAKDOWN VALUE, THE TRANSFER OF CHARGE BETWEEN SAID WEB AND SAID DRUM SURFACE THEREBY BEING INHIBITED; AND ELECTRICAL MEANS FOR MAINTAINING SAID CONDUCTIVE CYLINDER OF SAID DRUM AND THE OUTER SURFACE OF SAID SECOND GUIDE ROLLER AT SUBSTANTIALLY THE SAME POTENTIAL, WHEREBY THE ELECTROSTATIC IMAGE ON SAID PHOTOCONDUCTIVE LAYER IS TRANSFERRED TO SAID WEB. 